Grimmia longirostris was commonly recognized in China as Grimmia affinis Hornsch. [Flora 2(2): 443. 1819], which is a later homonym. The legitimate Grimmia affinis Hornsch. is a synonym of Grimmia fuscolutea Hook. The nomenclature and taxonomy of Grimmia longirostris were discussed in detail by Muñoz (1998a).

Grimmia longirostris is characterized by having keeled, lanceolate, hair-pointed leaves with margins recurved on one side below. Other features include: 1) the hyaline basal cells near the margins (at least on one side) are rectangular, with straight, thick transverse walls; 2) the juxtacostal cells are elongate and sinuose; 3) the setae are straight and the capsules are exserted, oblong-ovoid to cylindric; 4) the annuli are well developed; 5) the calyptrae are mitrate; and 6) the sexual condition is cladautoicous. Sometimes, the specimens of this species have been misnamed as G. ovalis (Hedw.) Lindb. or G. commutata Hüb. (nom. illeg.) in China as well as in the other parts of the world due to nomenclatural and taxonomic misunderstandings. According to Sayre (1951) and our observations, G. longirostris and G. ovalis are distinguishable. Grimmia ovalis has unkeeled and channeled leaves, with margins plane or incurved, cucullate calyptrae, and a dioicous sexual condition.

Grimmia longirostris in China shows a rather wide range of variation, especially in sporophytes. The following features are especially noteworthy: 1) The length of setae ranges from 2.0 to 4.5(–5.0) mm long, resulting in the capsules subimmersed when the setae are less than 2.5 mm long and exserted when the setae are more than 3.5 mm long. 2) The capsules show great variation in shape with their length varying from 1.5 to 2.8 mm and their width from 0.6 to 1.0 mm. Specimens with shorter setae (mostly from northern China) usually have ovoid capsules, while those with longer setae (mostly from southern and southwestern China) often have cylindrical capsules. 3) The beaks of opercula vary in relation to the capsule shape. The ovoid capsules usually have short, obtuse beaks, while the cylindrical capsules often have long, straight or slightly oblique beaks. 4) The lengths of hyaline hair-points of leaves are also variable. Usually, the leaves, especially the upper ones, have long hair-points, up to 1/3–1/4 the laminal length, but sometimes have rather short awns, only to 1/10 the laminal length. Geographically, the specimens with shortly awned leaves are scattered throughout the entire range of the species in China.